What wasn't quite explicit about BBC impartiality is now on the record, thanks to Lord Patten of Public Trust. "Some journalistic practices – such as paying for the information on MPs' expenses, as the Daily Telegraph did, or long-term heavily editorialised campaigns that are common in newspapers – would not be appropriate for the BBC, which is required to be impartial and must meet the highest standards of journalistic practice", he wrote in a letter to the Guardian.

Heavily editorialised campaigns? What are they, pray? Well, the chairman of the trust doesn't wrap that up too delphically. He was referring specifically to the Guardian's own "remorseless" pursuit of the phone-hacking story. Simple conclusion: revealing the hidden scandal of Westminster or the hidden scandal at the News of the World isn't an "appropriate" BBC activity. Investigations of uncaring care homes or Africa's killing fields are fine and lauded. But pause if you get too close to the heart of the system; ponder if you seem in risk of making huge waves.

Impartiality, in short, is a concept beginning to need a lot more discussion. The BBC employs thousands of terrific journalists. What stories can they do – and what revelations are somehow off-limits? If they can't campaign, what can they do?

Meanwhile, remember, quite counterintuitively, the BBC Trust just commissioned a special assessment of the corporation's reporting of the Arab spring from Edward Mortimer, lately UN director of communications for Kofi Annan. And who will be assessing Mr Mortimer's thoughts on BBC impartiality once concluded? Why, inter alia, the newest trustee on the block, appointed only last week, Michael Williams (aka Lord Williams of Baglan), former UN under-secretary general for the Middle East and special adviser to Kofi Annan. Somehow you don't see a great debate looming…